The UK terror threat level has been raised to its highest level of "critical", meaning further attacks may be imminent, Theresa May has said.
The move came after investigators were unable to rule out whether Manchester bombing suspect Salman Abedi acted alone, the prime minister said.
Military personnel will now be deployed to protect key sites.
Twenty-two people were killed and 59 injured when a suicide bomber attacked Manchester Arena on Monday evening.
The prime minister also confirmed the government had triggered "Operation Temperer", a long-standing emergency plan to put soldiers into key public locations to support armed police in protecting the public.
Military personnel may also be seen at other events over the coming weeks, such as concerts, Mrs May said, and would work under the command of police officers.
The prime minister said she did not want the public to feel "unduly alarmed" but said it was a "proportionate and sensible response".
She said the government would take "every measure available to us" to help the police protect the public.
"The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists.
"That is why the terrorists will never win and we will prevail," she said.